TYP: Overload series/ffill and bfill (pandas-dev#41152)

* TYP: Overload series/ffill and bfill

* Resolved comments

* Keep the return types

Author: pckSF

pandas/core/generic.py

@@ -6474,13 +6474,53 @@ def fillna(
         else:
             return result.__finalize__(self, method="fillna")
 
+    @overload
+    def ffill(
+        self: FrameOrSeries,
+        axis: None | Axis = ...,
+        inplace: Literal[False] = ...,
+        limit: None | int = ...,
+        downcast=...,
+    ) -> FrameOrSeries:
+        ...
+
+    @overload
+    def ffill(
+        self: FrameOrSeries,
+        axis: None | Axis,
+        inplace: Literal[True],
+        limit: None | int = ...,
+        downcast=...,
+    ) -> None:
+        ...
+
+    @overload
+    def ffill(
+        self: FrameOrSeries,
+        *,
+        inplace: Literal[True],
+        limit: None | int = ...,
+        downcast=...,
+    ) -> None:
+        ...
+
+    @overload
+    def ffill(
+        self: FrameOrSeries,
+        axis: None | Axis = ...,
+        inplace: bool_t = ...,
+        limit: None | int = ...,
+        downcast=...,
+    ) -> FrameOrSeries | None:
+        ...
+
     @final
     @doc(klass=_shared_doc_kwargs["klass"])
     def ffill(
         self: FrameOrSeries,
-        axis=None,
+        axis: None | Axis = None,
         inplace: bool_t = False,
-        limit=None,
+        limit: None | int = None,
         downcast=None,
     ) -> FrameOrSeries | None:
         """
@@ -6497,13 +6537,53 @@ def ffill(
 
     pad = ffill
 
+    @overload
+    def bfill(
+        self: FrameOrSeries,
+        axis: None | Axis = ...,
+        inplace: Literal[False] = ...,
+        limit: None | int = ...,
+        downcast=...,
+    ) -> FrameOrSeries:
+        ...
+
+    @overload
+    def bfill(
+        self: FrameOrSeries,
+        axis: None | Axis,
+        inplace: Literal[True],
+        limit: None | int = ...,
+        downcast=...,
+    ) -> None:
+        ...
+
+    @overload
+    def bfill(
+        self: FrameOrSeries,
+        *,
+        inplace: Literal[True],
+        limit: None | int = ...,
+        downcast=...,
+    ) -> None:
+        ...
+
+    @overload
+    def bfill(
+        self: FrameOrSeries,
+        axis: None | Axis = ...,
+        inplace: bool_t = ...,
+        limit: None | int = ...,
+        downcast=...,
+    ) -> FrameOrSeries | None:
+        ...
+
     @final
     @doc(klass=_shared_doc_kwargs["klass"])
     def bfill(
         self: FrameOrSeries,
-        axis=None,
+        axis: None | Axis = None,
         inplace: bool_t = False,
-        limit=None,
+        limit: None | int = None,
         downcast=None,
     ) -> FrameOrSeries | None:
         """